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Aluminum - Minerals and Uses
Aluminum is a silver-white metal, very light in weight (less than three times as dense as water), yet relatively strong. In addition, aluminum is ductile, that is, it can be drawn into wires or pressed into sheets or foil. It is the most abundant metallic element, and the third most abundant of all elements in the Earth’s crust, making up 8% of the crust by weight. Only silicon and oxygen are more plentiful.
Aluminum has numerous applications in the home and industry, and is a familiar metal to nearly everyone.
Aluminum is a reactive metal, and does not occur in the metallic state in nature. Therefore, it was unknown as a separate element until the 1820’s, although its existence was predicted by several scientists who had studied aluminum compounds. The name aluminum was derived from alumen, the Latin name for alum (an aluminum sulfate mineral).
Because aluminum metal reacts with water and air to form powdery oxides and hydroxides, aluminum metal is never found in nature. Many common minerals, including feldspars, contain aluminum, but extracting the metal from most minerals is very energy-intensive, and therefore expensive.
The main ore of aluminum is bauxite, the source of over 99% of metallic aluminum. Bauxite is the name for a mixture of similar minerals that contain hydrated aluminum oxides. These minerals are gibbsite (Al(OH)3), diaspore (AlO(OH)), and boehmite (AlO(OH)). Because it is a mixture of minerals, bauxite itself is a rock, not a mineral. Bauxite is reddish-brown, white, tan, and tan-yellow. It is dull to earthy in luster and can look like clay or soil. Bauxite forms when silica in aluminum-bearing rocks (that is, rocks with a high content of the mineral feldspar) is washed away (leached). This weathering process occurs in tropical and subtropical weathering climates.
Alternative sources of aluminum might someday include kaolin clay, oil shales, the mineral anorthosite, and even coal waste. However, as long as bauxite reserves remain plentiful and production costs are low, the technologies to process these alternative sources into alumina or metallic aluminum will likely not progress beyond the experimental stage.
Australia has huge reserves of bauxite, and produces over 40% of the world’s ore. Brazil, Guinea, and Jamaica are important producers, with lesser production from about 20 other countries. Recycling of aluminum by melting cans and other products is an important source of metal in many developed countries.
About 85% of all the bauxite mined worldwide is used to produce alumina for refining into aluminum metal. Another 10% produces alumina which is used in chemical, abrasive, and refractory products. The remaining 5% of bauxite is used to make abrasives, refractory materials, and aluminum compounds.
The lightness, strength, and corrosion resistance of aluminum are important considerations in its application. Metallic aluminum is used in transportation, packaging such as beverage cans, building construction, electrical applications, and other products.
There are possible replacements for aluminum but unless energy costs should rise steeply, the use of aluminum in most of these applications is not likely to be seriously threatened. Worldwide sources of bauxite are large enough to supply the demand for aluminum for some time to come.